[llvm-branch-commits] [llvm] release/23.x: RISCV: Fix optimizeCondBranch crash on an undef register (#216633) (PR #216645)

[via llvm-branch-commits]

https://github.com/llvmbot created https://github.com/llvm/llvm-project/pull/216645

Backport 38da688c60b208a48d60b70396ac46716edc72d5

Requested by: @wangpc-pp

>From efe73f21f7e92e59a1f67a7d39e2be61fd81d966 Mon Sep 17 00:00:00 2001
From: Matt Arsenault <Matthew.Arsenault at amd.com>
Date: Mon, 17 Aug 2026 07:46:37 +0200
Subject: [PATCH] RISCV: Fix optimizeCondBranch crash on an undef register
 (#216633)

Found by AI while working on something else.

Co-authored-by: Claude (Opus 4.8) <noreply at anthropic.com>
(cherry picked from commit 38da688c60b208a48d60b70396ac46716edc72d5)
---
 llvm/lib/Target/RISCV/RISCVInstrInfo.cpp      |  7 +++-
 .../RISCV/optimize-cond-branch-undef.mir      | 36 +++++++++++++++++++
 2 files changed, 42 insertions(+), 1 deletion(-)
 create mode 100644 llvm/test/CodeGen/RISCV/optimize-cond-branch-undef.mir

diff --git a/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp b/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
index e36410fbcb593..49d8fa0fc27fd 100644
--- a/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
+++ b/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
@@ -1622,7 +1622,12 @@ bool RISCVInstrInfo::isFromLoadImm(const MachineRegisterInfo &MRI,
     Imm = 0;
     return true;
   }
-  return Reg.isVirtual() && isLoadImm(MRI.getVRegDef(Reg), Imm);
+
+  if (!Reg.isVirtual())
+    return false;
+
+  const MachineInstr *DefMI = MRI.getVRegDef(Reg);
+  return DefMI && isLoadImm(DefMI, Imm);
 }
 
 bool RISCVInstrInfo::optimizeCondBranch(MachineInstr &MI) const {
diff --git a/llvm/test/CodeGen/RISCV/optimize-cond-branch-undef.mir b/llvm/test/CodeGen/RISCV/optimize-cond-branch-undef.mir
new file mode 100644
index 0000000000000..2c291b5f3a120
--- /dev/null
+++ b/llvm/test/CodeGen/RISCV/optimize-cond-branch-undef.mir
@@ -0,0 +1,36 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 6
+# RUN: llc -mtriple=riscv64 -run-pass=peephole-opt -o - %s | FileCheck %s
+
+# A conditional branch on an undef register with no def must not crash the
+# peephole optimizer while it checks whether the operand is a load-immediate.
+---
+name: cond_branch_undef
+tracksRegLiveness: true
+body: |
+  ; CHECK-LABEL: name: cond_branch_undef
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.1(0x40000000), %bb.2(0x40000000)
+  ; CHECK-NEXT:   liveins: $x10
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[COPY:%[0-9]+]]:gpr = COPY $x10
+  ; CHECK-NEXT:   BLT undef %1:gpr, [[COPY]], %bb.1
+  ; CHECK-NEXT:   PseudoBR %bb.2
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   PseudoRET
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.2:
+  ; CHECK-NEXT:   PseudoRET
+  bb.0:
+    successors: %bb.1, %bb.2
+    liveins: $x10
+    %0:gpr = COPY $x10
+    BLT undef %1:gpr, %0, %bb.1
+    PseudoBR %bb.2
+
+  bb.1:
+    PseudoRET
+
+  bb.2:
+    PseudoRET
+...